This invention relates to dispersion-shifted optical fiber having large effective core area and low dispersion slope.
The present invention is based on patent applications filed in Japan (Japanese Patent Application No. H11-212949/1999, Japanese Patent Application No. H11-230137/1999, Japanese Patent Application No. 2000-64008, Japanese Patent Application No. 2000-224491, and Japanese Patent Application No. 2000-224492), and the particulars described in those Japanese patent applications are incorporated as part of this specification.
In a long-haul system such as an optical amplifier repeater transmission system employing optical fiber amplifiers, it is important that the nonlinear optical effects be reduced. A parameter called the nonlinear optical coefficient is a parameter that serves as an index to the degree of nonlinear optical effect. The nonlinear optical coefficient is expressed by n2/Aeff, where n2 is the nonlinear refractive index and Aeff is the effective core area. The value of n2 becomes roughly constant depending on the material, and expanding the Aeff is an effective technique for reducing nonlinear optical effects.
In wavelength division multiplexed transmission systems which can handle high-volume transmission, on the other hand, there is a need to suppress the chromatic dispersion value and reduce dispersion slope. It is well known that, in a wavelength division multiplexed transmission system, when a zero-dispersion wavelength exists in the transmission bandwidth, transmission quality declines due to a nonlinear optical effect called four-wave mixing. However, because large chromatic dispersion values are accompanied by signal waveform deterioration, it is necessary to suppress that value to a certain size. In order to satisfy these conflicting demands, optical fiber called non-zero dispersion-shifted optical fiber in which the chromatic dispersion value in the wavelength band used is controlled to within a narrow range has been developed.
In a wavelength division multiplexed transmission system, furthermore, reducing the dispersion slope is also important. By dispersion slope, which indicates the wavelength dependency of the chromatic dispersion value, is meant the slope of the curve obtained by plotting wavelength (nm) on the horizontal axis and chromatic dispersion value (ps/kmxc2x7nm) on the vertical axis. In a wavelength division multiplexed transmission system, if the dispersion slope of the transmission line (optical fiber) is large, the difference in chromatic dispersion value between wavelengths will be great. For that reason, by taking a very large dispersion value, depending on the wavelength, difficulties are encountered such as the transmission quality being greatly different between different channels. Accordingly, there is a need to make the dispersion slope smaller.
The specific values for the characteristics demanded in the Aeff and dispersion discussed above will be different according to the system employed. In a system in which transmissions are made over very long haul, such as submarine systems, a reduction in nonlinear optical effect resulting from Aeff expansion is sought. In a system extending for from several tens to several hundreds of km, on the other hand, there is sometimes a need to suppress the dispersion value in a wide wavelength band by dispersion slope reduction. In terms of the minimum conditions demanded for the transmission line in a light communication system, furthermore, the optical fiber should be substantially single-mode, and the bending loss should be held down to 100 dB/m or lower.
That being so, proposals have recently been made on ways to effect some degree of Aeff expansion and dispersion slope decrease using various refractive index distribution shapes (refractive index profiles), as in Japanese Patent Application Laid-Open No. H10-62640/1998, Japanese Patent Application Laid-Open No. H10-293225/1998, Japanese Patent Application Laid-Open No. H8-220362/1996, and Japanese Patent Application Laid-Open No. H10-246830/1998, for example.
In FIGS. 10A to 10C are diagrammed examples of shapes of refractive index distribution of such dispersion-shifted optical fiber.
In FIG. 10A is represented one example of a dual shape core (step) type of refractive index distribution, in a core 14 is formed, with the symbol 11 designating the center core portion and a step core portion 12 provided about the outer circumference thereof having a lower refractive index than the center core portion 11. Furthermore, about the outer circumference of that core 14, clad 17 is provided having a lower refractive index than the step core portion 12.
In Japanese Patent Application Laid-Open No. H8-220362/1996, the present applicant disclosed the use of the smaller diameter solution, for the purpose of expansion of Aeff, in dispersion-shifted optical fiber having a dual shape core type refractive index distribution.
It has been known for some time that, when the core diameter of a dispersion-shifted optical fiber is expanded while maintaining the similarity of refractive index distribution shape, two or more solutions exist wherewith the chromatic dispersion value becomes the desired value. At such time, of the solutions within a range wherein the bending loss and cutoff wavelength characteristics are comparatively practical, the solution wherewith the core diameter is relatively thin is called the smaller diameter solution, and the solution wherewith the core diameter is relatively large is called the larger diameter solution.
In FIG. 10B is represented an example of a segmented core type of refractive index distribution shape, wherein a core 24 is configured with an intermediate portion 22 of low refractive index provided about the outer circumference of the center core portion 21 of high refractive index, and a ring core portion 23 having a higher refractive index than the intermediate portion 22 but a lower refractive index than the center core portion 21 provided about the outer circumference of that intermediate part 22. Also, about the outer circumference of that ring core portion 23 is provided a first clad 25 having a lower refractive index than the intermediate portion 22, and about the outer circumference of that first clad 25 is provided a second clad 26 having a higher refractive index than the first clad 25 but a lower refractive index than the intermediate portion 22, thus configuring clad 27.
In Japanese Patent Application Laid-Open No. H11-119045/1999 (published), furthermore, the present applicant disclosed a dispersion-shifted optical fiber well suited to optical communication systems wherein the reduction of the dispersion slope is more rigorously demanded than the expansion of the Aeff, by using the larger diameter solution in a segmented core type of refractive index distribution shape.
In FIG. 10C is represented an example of an O ring type refractive index distribution shape, wherein a core 34 is configured with a two-layer structure, with a peripheral core portion 32 of high refractive index provided about the outer circumference of a center core portion 31 of low refractive index at the center. About the outer circumference of that core 34 is provided clad 37 of lower refractive index than the peripheral core portion 32, thereby configuring a three-layer structure convex type refractive index distribution shape inclusive of the clad 37.
In the dispersion-shifted optical fibers conventionally proposed, however, under such conditions as that they are substantially single-mode and that the bending loss is held down to 100 dB/m or lower, it is very difficult to sufficiently realize both Aeff expansion and dispersion slope reduction simultaneously.
Looking at the dual shape core type of optical fiber wherein the smaller diameter solution is used disclosed in Japanese Patent Application Laid-Open No. H8-220362/1996, for example, the dispersion slope is in the neighborhood of 0.10 ps/km/nm2 at minimum, wherefore this optical fiber is sometimes inadequate for use in systems where dispersion slope reduction is rigorously demanded.
With the segmented core type optical fiber wherewith the larger diameter solution is used, disclosed in Japanese Patent Application Laid-Open No. H11-119045/1999, characteristics close to those demanded in somewhat more recent wavelength division multiplexed transmission systems are obtained. However, because the refractive index distribution shape comprises a five-layer structure wherein the refractive index increases and declines, the characteristics vary subtly depending on the position, width, and shape, etc., of each layer. Accordingly, during manufacture, a high level of controllability is demanded for such structural parameters as the radius and relative refractive-index difference of each layer. As a consequence, there is a limit to the degree to which product yield can be improved.
With the increase in the number of channels (i.e. number of multiplexed wavelengths), moreover, dispersion-shifted optical fiber has come to be demanded which can be employed all across a wide transmission wavelength band of 1490 to 1625 nm in which the so-called L band (1570 to 1610 nm) has been added.
Conventional dispersion-shifted optical fiber of expanded Aeff is designed with transmission in the 1550 nm band in view, wherefore such optical fiber having adequate characteristics in the L band has not been provided. In many cases, bending loss became large particularly in the L band.
An object of the present invention, which was devised in view of the circumstances described above, is to provide dispersion-shifted optical fiber wherewith Aeff expansion and dispersion slope reduction can both be satisfactorily realized, simultaneously, under conditions such that a single-mode is substantially realized and the bending loss is held to 100 dB/m or less.
Another object is to provide dispersion-shifted optical fiber wherewith stabilized characteristics are exhibited with a structure made as simple as possible, that can nevertheless be efficiently manufactured.
Another object is to provide dispersion-shifted optical fiber wherewith Aeff expansion and dispersion slope reduction can both be satisfactorily realized, simultaneously, under conditions such that a single-mode is substantially realized and the bending loss is held to 100 dB/m or less, even in a broad wavelength band to which the L band has been added, covering from 1490 to 1625 nm.
Another object is to provide dispersion-shifted optical fiber that exhibits low bending loss, particularly in the L band.
In order to realize the objects stated above, a first dispersion-shifted optical fiber of the present invention is dispersion-shifted optical fiber having a refractive index distribution shape comprising a center core portion of high refractive index, a step core portion of lower refractive index than the center core portion, provided about the outer circumference thereof, and clad of lower refractive index than the step core portion, provided about the outer circumference of the step core portion, in which, the dispersion-shifted optical fiber has, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to 90 xcexcm2, dispersion slope of from 0.05 to 0.14 ps/km/nm2, bending loss of 100 dB/m or less, and chromatic dispersion value of either from xe2x88x920.5 to xe2x88x928.0 ps/km/nm or from +0.05 to +10.0 ps/km/nm, and has a cutoff wavelength such that substantially single-mode propagation is realized.
A second dispersion-shifted optical fiber is characterized in that, in the first dispersion-shifted optical fiber, larger diameter solution is adopted for core diameter, and the dispersion-shifted optical fiber has, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to 70 xcexcm2, dispersion slope of from 0.05 to 0.08 ps/km/nm2, bending loss of 100 dB/m or less, and chromatic dispersion value of from xe2x88x920.5 to xe2x88x928.0 ps/km/nm, and has a cutoff wavelength such that substantially single-mode propagation is realized.
A third dispersion-shifted optical fiber is characterized in that, in the second dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, relative refractive-index difference of the center core portion when refractive index of outermost clad is taken as reference as xcex941, and relative refractive-index difference of the step core portion as xcex942, r2/r1 is from 4 to 12, xcex942/xcex941 is from 0.05 to 0.15, and xcex941 is from 0.55 to 0.85%.
A fourth dispersion-shifted optical fiber is characterized in that, in the second dispersion-shifted optical fiber, the clad comprises first clad provided about outer circumference of said step core portion and second clad having a higher refractive index than the first clad, provided about outer circumference of the first clad.
A fifth dispersion-shifted optical fiber is characterized in that, in the fourth dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, radius of the first clad as r3, relative refractive-index difference of the center core portion when refractive index of the outermost clad is taken as reference as xcex941, relative refractive-index difference of the step core portion as xcex942, and relative refractive-index difference of the first clad as A3, r2/r1 is from 4 to 12, xcex942/xcex941 is from 0.05 to 0.15, xcex941 is from 0.55 to 0.85%, xcex943 is from xe2x88x920.3 to 0%, and (r3xe2x88x92r2)/r1 is from 0.2 to 4.0.
A sixth dispersion-shifted optical fiber is characterized in that, in the first dispersion-shifted optical fiber, larger diameter solution is adopted for core diameter, and the dispersion-shifted optical fiber has, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to 70 xcexcm2, dispersion slope of from 0.05 to 0.075 ps/km/nm2; bending loss of 100 dB/m or less, and chromatic dispersion value of from +0.05 to +10.0 ps/km/nm, and has a cutoff wavelength such that substantially single-mode propagation is realized.
A seventh dispersion-shifted optical fiber is characterized in that, in the sixth dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, relative refractive-index difference of the center core portion when refractive index of the outermost clad is taken as reference as xcex941, and relative refractive-index difference of the step core portion as xcex942, r2/r1 is from 4 to 12, xcex941 is from 0.55 to 0.75%, and xcex942/xcex941 is from 0.05 to 0.15.
An eighth dispersion-shifted optical fiber is characterized in that, in the sixth dispersion-shifted optical fiber, the clad comprises first clad provided about outer circumference of the step core portion and second clad provided about outer circumference thereof.
A ninth dispersion-shifted optical fiber is characterized in that, in the eighth dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, radius of the first clad as r3, relative refractive-index difference of the center core portion when refractive index of the second clad is taken as reference as xcex941, relative refractive-index difference of the step core portion as xcex942, and relative refractive-index difference of the first clad as xcex943, r2/r1 is from 4 to 12, xcex941 is from 0.55 to 0.75%, xcex942/xcex941 is from 0.05 to 0.15, xcex943 is from xe2x88x920.1 to 0%, and (r3xe2x88x92r2)/r1 is from 0.2 to 4.0.
A tenth dispersion-shifted optical fiber is characterized in that, in the first dispersion-shifted optical fiber, smaller diameter solution is adopted for the core diameter, and the dispersion-shifted optical fiber has, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 65 to 95 xcexcm2, dispersion slope of from 0.08 to 0.14 ps/km/nm2, bending loss of 100 dB/m or less, and the absolute values of the chromatic dispersion value of from 0.5 to 8.0 ps/km/nm, and has a cutoff wavelength such that substantially single-mode propagation is realized.
An 11th dispersion-shifted optical fiber is characterized in that, in the tenth dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, relative refractive-index difference of the center core portion when refractive index of the clad is taken as reference as xcex941, relative refractive-index difference of the step core portion as xcex942, r2/r1 as x, and xcex942/xcex941 as y, 5xe2x89xa6xxe2x89xa610, 0.08xe2x89xa6yxe2x89xa60.22, and 0.6%xe2x89xa6xcex941xe2x89xa61.2%.
A 12th dispersion-shifted optical fiber is the tenth dispersion-shifted optical fiber having a zero dispersion wavelength on the side of longer wavelengths than the wavelength band used.
A 13th dispersion-shifted optical fiber is characterized in that, in the 12th dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, relative refractive-index difference of the center core portion when refractive index of the clad is taken as reference as xcex941, relative refractive-index difference of the step core portion as xcex942, r2/r1 as x, and xcex942/xcex941 as y, 6xe2x89xa6xxe2x89xa67, 0.1xe2x89xa6yxe2x89xa60.18, yxe2x89xa7(xe2x88x920.02x+0.24), 0.6%xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 65 to 75 xcexcm2, and dispersion slope is 0.125 ps/km/nm2 or less.
A 14th dispersion-shifted optical fiber is characterized in that, in the 12th dispersion-shifted optical fiber, 7xe2x89xa6xxe2x89xa68, 0.1xe2x89xa6yxe2x89xa60.16, yxe2x89xa7(xe2x88x920.016x +0.21), 0.6%xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 70 to 80 xcexcm2, and dispersion slope is 0.130 ps/km/nm2 or less.
A 15th dispersion-shifted optical fiber is characterized in that, in the 12th dispersion-shifted optical fiber, 7xe2x89xa6xxe2x89xa68.5, 0.1xe2x89xa6yxe2x89xa60.16, (xe2x88x920.02x+0.26)xe2x89xa6yxe2x89xa6(xe2x88x920.02x+0.32), 0.6%xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 75 to 85 xcexcm2, and dispersion slope is 0.135 ps/km/nm2 or less.
A 16th dispersion-shifted optical fiber is the tenth dispersion-shifted optical fiber having a zero dispersion wavelength on the side of shorter wavelength than the wavelength band used.
A 17th dispersion-shifted optical fiber is characterized in that, in the 16th dispersion-shifted optical fiber, when radius of the center core portion is represented as r1, radius of the step core portion as r2, relative refractive-index difference of the center core portion when refractive index of the clad is taken as reference as xcex941, relative refractive-index difference of the step core portion as xcex942, r2/r1 as x, and xcex942/xcex941 as y, 5xe2x89xa6xxe2x89xa68, 0.12xe2x89xa6yxe2x89xa60.22, (xe2x88x920.02x+0.24)xe2x89xa6yxe2x89xa6(xe2x88x920.02x+0.34), 0.6%xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 65 to 75 m2, and dispersion slope is 0.110 ps/km/nm2 or less.
An 18th dispersion-shifted optical fiber is characterized in that, in the 16th dispersion-shifted optical fiber, 5.5xe2x89xa6xxe2x89xa68, 0.12xe2x89xa6yxe2x89xa60.20, (xe2x88x920.02x+0.25)xe2x89xa6yxe2x89xa6(xe2x88x920.02x+0.33), 0.6%xe2x89xa6xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 70 to 80 xcexcm2, and dispersion slope is 0.115 ps/km/nm2 or less.
A 19th dispersion-shifted optical fiber is characterized in that, in the 16th dispersion-shifted optical fiber, 6xe2x89xa6xxe2x89xa68, 0.12xe2x89xa6yxe2x89xa60.20, (xe2x88x920.02x+0.26)xe2x89xa6yxe2x89xa6(xe2x88x920.02x+0.35), 0.6%xe2x89xa6xcex941xe2x89xa61.2%, Aeff is from 75 to 85 xcexcm2, and dispersion slope is 0.125 ps/km/nm2 or less.